3. Underhill House, Gloucestershire

Architect Helen Seymour-Smith was given the plot for Underhill House by her father. Situated in an area of outstanding natural beauty, the plot included a dilapidated 300-year-old stone barn. The catch? To date all development proposals had been refused, the planners reluctant to spoil the bucolic view of the honey-coloured barn amidst gentle rolling fields.

Bold and indefatigable, Seymour-Smith's design for Underhill House went on to win the approval of the planners – one of only 20 properties in the UK ever to be granted planning permission in an area of outstanding natural beauty. It was also the first certified Passivhaus in England.

The north side of the home is buried underground, drawing warmth from the earth. Photograph: Samuel Ashfield, Seymour-Smith Architects, Dow / Sto

Almost completely hidden behind the comparably conventional barn conversion, Underhill House burrows underground and is near invisible on the landscape, allowing Seymour-Smith to build the family home she wanted. "We want to bring the Italian catwalk to eco houses," she wrote at the beginning of the build, "proving that you don't have to knit your own sandals to be an environmentally conscious builder."

But this is no dark and dingy warren. Expansive south-facing triple-glazing creates the perfect passive solar design whilst the north side of the home is buried underground, drawing warmth from the earth and eliminating the need for a central heating system.

The bedrooms are arranged to receive as much natural light as possible. Photograph: Samuel Ashfield, Seymour-Smith Architects, Dow / Sto

The sleek structure of the house – which was built for a total of £575,000 – is entirely concrete, much of which is left exposed internally. The galvanised steel ventilation ductwork and wiring conduits are laid bare creating a minimal, industrial aesthetic a million miles away from Bag End. The four bedrooms are on a single level, and each are arranged to receive as much natural light as possible. There are dropped ceiling panels to aid acoustics and segregate the expansive open-plan living area and internal walls are, of course, almost all white to bounce light through the home.

It's like "loft living underground", according to Seymour-Smith. Loft living in the heart of the glorious English countryside.